A boat is often used in semiconductor manufacturing processes as a carrier for holding and transporting a plurality of semiconductor devices, such as wafers. The boats are transported using various types of specialized handling equipment, such that an entire batch of wafers carried on a boat is processed at the same time. One particular set of handling equipment is employed to move a boat between a standby or transfer position, to a loading position from which equipment, through a series of mechanized movements, transports the boat into a processing station, such as a furnace, for example, where a diffusion operation is performed. Following the simultaneous processing of all the wafers carried on a given boat in a single batch processing operation, the boat is withdrawn in a reverse set of movements by the handling equipment to a transfer station where the boat is transported by other transport mechanisms to another processing station.
The boat handling equipment referred to above typically comprises a boat transfer unit, a boat elevator, a boat clamp and a boat inserter controller. The boat transfer unit, which includes a transfer arm on which the boat sits, transfers the boat from the wafer transfer position, to the loading position vertically aligned above the boat elevator. The boat elevator moves upwardly to contact and engage the boat, whereupon the boat is clamped to the elevator which then moves the boat upwardly into a process chamber. The boat transfer arm typically includes a series of stops and alignment pins which help locate and stabilize the boat on the arm, however the boat is not securely clamped to the arm, and thus is subject to tipping as a result of numerous influences such as vibrations or jolting caused by the equipment or by events such as earth tremors.
Since the boat transfer sequence, like most operations during the fabrication process, is carried out completely automatically by computer controlled robotic mechanisms, it has not been possible to detect when a boat has tipped on a transfer arm to any degree. As a result, where tipping forces are severe, the entire boat may tip over during the transfer process, thus resulting in damage to the wafers, equipment down time and related reduction in throughput. Even where forces causing a boat to tip are not severe, the tipping motion may be sufficient to move the boat out of its normal on-axis alignment position on the transfer arm. This slight off-axis tilt of the boat can result in the boat not moving squarely into the process chamber, but rather colliding with the chamber walls or a scavenger cover disposed adjacent one end of the chamber. This collision, again, can result in process interruption requiring human intervention, as well as wafer damage.
It is apparent then that there is a clear need in the art for apparatus for both sensing when a boat has tipped beyond a threshold level requiring attention, as well as for preventing severe tipping or toppling over of a boat on a transfer arm.